Discharge lamps are used in many fields of application where characteristics of high luminance and quality of the radiation are required, for example in cinematographic and television filming, in the photographic field, in scientific and medical applications, and as sources of light in a large range of projectors. Recent developments have introduced such lamps in the automotive market.
During operation such lamps are supplied by a voltage of the order of several tens or hundreds of volts (supply voltage) which can maintain an electric arc between the electrodes of the lamp. Depending on the model of the lamp the supply voltage required is dc, ac (in the majority of actual cases at mains frequency) or a square wave.
Upon starting it is necessary to strike the arc between the electrodes of the lamp in such a way that it can then be maintained over time by the supply voltage. In the cold state discharge lamps are perfect insulators since the gas filling the bulb has an insulating effect between the two electrodes, and the application of only the supply voltage (from 15 to 440 volts depending on the type of lamp) does not produce sufficient energy to strike the arc. For starting the lamp, therefore, it is necessary to ionise and render conductive the gas filling the bulb through a voltaic arc caused by a high voltage discharge.
Discharge lamps are provided with igniter devices (also simply called "igniters") able to provide a high ignition voltage, of the order of kilovolts, to the electrodes. Although only a few kilovolts are needed for starting when the lamps are cold, to obtain ignition in the hot state after the lamp has been extinguished and the dielectric resistance to discharge is greater, voltages up to 10 times greater (10-70 kV) are necessary.
To allow lamps to be started in all temperature conditions it is known in the art to use "superimposition" igniters, the mode of operation of which consists in superimposing, during the ignition phase, the discharge ignition voltage over the supply voltage provided for normal operation. The ignition voltage required is provided in the form of a sequence of pulses obtained as a consequence of the electrical discharge across the terminals of a spark gap, amplified by a transformer coupling the igniter with a supply circuit of the lamp. The ignition voltage depends in such cases on the frequency, and voltages of the order of several tens of kilovolts such as those necessary for igniting the lamp in any temperature state are in practice obtained by means of igniters which generate pulses at frequencies in the megahertz range. A typical pulse generated across the terminals of the lamp by a superimposition igniter with a spark gap operating in the megahertz range is represented in attached FIG. 5, in which the damped oscillations are reproduced in the drawing at a frequency not representative of the frequency of oscillation in real cases solely for the purpose of clarity of illustration.
Although by now established and reliable, the ignition technique using igniters provided with spark gaps has intrinsic disadvantages, above all from the point of view of the control of the operation. The discharge through the spark gap, constitutes an electrical phenomenon which is generally difficult to control, the parameters of which can vary as a function, for example, of the deterioration of the spark gap itself which must periodically be replaced or adjusted. The discharge to the electrodes is a disruptive discharge having many harmonic components, and is therefore a source of electromagnetic and acoustic noise. Since lamps of the type described are commonly used in environments where electronic circuits sensitive to electromagnetic noise are present, it is important to have available igniter devices which disturb the operation of nearby circuits as little as possible. In a spark gap the electrical discharge across the terminals of the electrodes is, moreover, a phenomenon the instant of activation of which is difficult to control with precision, and the duration of which is severely limited; since the spark gap consumes a considerable amount of energy the current between the electrodes falls rapidly and the electric arc is interrupted after a short time (the duration T.sub.d shown is, in practical cases, about 5 .mu.s).
In U.S. Pat. No. 4,060,751 entitled "Dual Mode Solid State Inverter Circuit for Starting and Ballasting Gas Discharge Lamps" published Nov. 29, 1997, there is described an inverter circuit comprising a pair of solid state switches operating in phase opposition used for the production of an igniter/power supply for discharge lamps. A resonant circuit is associated with the lamp and controlled by the inverter circuit at the resonant frequency so as to generate peak voltages across the terminals of the discharge lamp and facilitate ignition thereof. The quality factor Q of the resonant circuit is however very low (a Q greater than 2 or 3 is considered to be sufficient) and the ignition voltage which is generated across the electrodes of the lamp cannot reach values of tens of kilovolts. In the particular circuit arrangement, after starting of the lamp the igniter acts as a supply circuit thereof and limits the supply current thereto. However this arrangement does not provide the necessary versatility and, in particular, the igniter described is not easily controllable to manage the number and duration of pulses as a function of the type of lamp to which it is coupled.